1. Field of the Invention
This invention relates to liquid electrophotography, especially an apparatus and method for removing carrier liquid from an intermediate transfer member surface or from a liquid developed toned image on an intermediate transfer member.
2. Background of the Art
Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoreceptor, imagewise exposing the photoreceptor to activating electromagnetic radiation (also referred to herein as xe2x80x9clightxe2x80x9d and including infrared, visible light and ultraviolet radiation) and thereby dissipating the charge in the exposed areas to form an electrostatic latent image, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoreceptor to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. The direct or intermediate transfer typically occurs by one of two methods: electrostatic assist (electrostatic transfer) or elastomeric assist (adhesive transfer). xe2x80x9cAdhesive transferxe2x80x9d means that transfer was primarily effected by surface tension phenomena (e.g., including tack) between the receptor surface and the temporary carrier surface or medium for the toner. xe2x80x9cElectrostatic transferxe2x80x9d means that transfer was primarily effected by electrostatic charges or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner.
The effectiveness of adhesive transfer is controlled by several variables including surface energy, temperature, and pressure. Electrostatic transfer is also affected by surface energy, temperature, and pressure, but the primary driving force causing the toner image to be transferred to the final substrate is via electrostatic forces.
The structure of a photoreceptor generally may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. All photoreceptors have a photoconductive layer which transports charge (either by an electron transfer of charge transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoreceptor is either negatively or positively charged such that when activating electromagnetic radiation strikes a region of the photoconductive layer, charge is conducted through the photoreceptor in that region to neutralize, dissipate or reduce the surface potential in the illuminated region. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and extend the service life of the photoconductive layer. Other layers, such as adhesive layers or priming layers or charge injection blocking layers are also used in some photoreceptors. A release layer may be used to facilitate transfer of the image from the photoreceptor to either the final substrate, such as paper, or to an intermediate transfer element.
Typically, a positively charged toner is attracted to those areas of the photoreceptor which retain a negative charge after the imagewise exposure, thereby forming a toner image which corresponds to the electrostatic latent image. The toner need not be positively charged, although that charge form or a neutral charge is preferable. Some toners (irrespective of their charge) may be attracted to the areas of the photoreceptor where the charge has been dissipated. The toner may be either a powdered material comprising a blend or association of polymer and colored particulates, typically carbon for a black image, or a liquid material of finely divided solids dispersed in an insulating liquid that is frequently referred to as a carrier liquid.
Generally, the carrier liquid is a hydrocarbon that has a low dielectric constant (e.g., less than 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image. Examples of such carrier liquids include NORPAR(trademark) and ISOPAR(trademark) solvents from Exxon Chemical Company.
Liquid toners are often preferable because they are capable of giving higher resolution images and require lower energy for image fixing than do dry toners. However, excess carrier liquid which is transferred to the photoreceptor can create a variety of problems. When either the elastomeric or adhesive transfer mechanism is being used, removal of excess carrier liquid is especially important. The excess carrier liquid can blot or stain the image or can cause smudging or streaking of the images. In addition, if excess carrier liquid is not removed, additional energy will be required at the image fixing step to volatilize the excess carrier liquid. Also, removal of the excess carrier liquid generally leads to improved image clarity and image density.
A variety of methods have been employed to remove excess carrier liquid from a developed toner image. These methods include squeegee rolls, air knives, corona discharge, vacuum removal, and absorption.
U.S. Pat. No. 5,420,675 to Thompson et al. discloses the use of a film forming roller which has a thin, outer layer which is compatible (referred to as xe2x80x98phillicxe2x80x99) with the carrier liquid and an inner layer which is carrier liquid-phobic and compressible. The film forming roller of that patent is maintained in contact with a single heating roller. The carrier liquid entrained in the film forming roller is removed by heating the liquid to a temperature greater than or equal to the flashpoint of the liquid.
U.S. Pat. No. 5,552,869 to Schilli et al. discloses a drying method and apparatus for electrophotography using liquid inks. The drying apparatus removes excess carrier liquid from an image produced by liquid electrophotography on a moving organophotoreceptor. The system includes a drying roller that contacts the organophotoreceptor, with an outer layer that absorbs and desorbs the carrier liquid and an inner layer having a Shore A hardness of 10 to 60 which is carrier liquid-phobic, and a heating means to increase the temperature of the drying roller to no more than 5xc2x0 C. below the flash point of the carrier liquid. In one embodiment, the heating means includes two hot rollers and the system further includes a cooling means that cool the drying roller.
U.S. Pat. No. 5,736,286 to Kaneko et al. discloses the employment of a drying belt to remove carrier fluids in liquid inks.
The art teaches drying of a liquid toned image by absorbing or xe2x80x9cdryingxe2x80x9d processes consisting of absorbing the excess carrier fluid from the image face, after the image is plated onto the photoreceptor and before the image is transferred to the receiving medium, by means of an absorptive polymer layer coated onto a roller, belt, or disk. Other methods of carrier fluid removal include: drying the image from the backside of the image using vacuum assistance through a semi-permeable membrane; thermally drying the receiving medium after the image has been transferred, absorbing by the drying member, of excess carrier fluid from a non-absorptive intermediate transfer belt after the image has been transferred to the receiving medium; and thermally evaporating the excess carrier fluid from an absorptive transfer belt and/or the image into the surrounding environment.
In cases of continuous printing, the drying belt or roller may become substantially saturated with carrier liquid at some point. At this point, regeneration or xe2x80x9crenewingxe2x80x9d the drying member is desirable because absorption of carrier fluid by the drying member may be repeated after the carrier has been absorbed and the imaging cycle completed. Regeneration is usually facilitated by heat, pressure, or vacuum or a combination thereof. After regeneration is completed, the drying member is capable of absorbing more carrier fluid because the drying member remains unsaturated with the carrier fluid. The art teaches thermal regeneration of the drying member to prevent saturation; however, this use of a single belt or roller, which is subject to countless absorption and regeneration (or xe2x80x9cdesorptionxe2x80x9d) cycles, has many associated problems.
For example, continual contact of a heated regeneration element with the drying belt or roller can cause the belt or roller to heat up, resulting in hot offset of the liquid toned image to the hot surface of the drying belt or roller.
Another problem is associated with the need for constant absorption and regeneration of the drying member. A heating element is frequently used to evaporate the unwanted carrier liquid in the drying roller or belt. This evaporation step creates carrier liquid vapor that may be harmful to consumers and may be regulated by environmental standards. The harmful vapor must therefore be collected or rendered harmless creating a need for a vapor collection system or apparatus in the imaging line (a complex and usually costly system typically comprising at least a fan, collection ducts, and a condenser). The evaporated and condensed carrier is then stored in liquid form in the printer until disposal.
Another problem that occurs in carrier removal is that the repetitive use and regeneration of the same drying or absorptive belt or roller degrades the absorbent layer, introducing artifacts/contaminants to the toner image, and generally decreasing the life of the drying roller or belt. The high heat necessary to continually evaporate a non-volatile or high flashpoint solvent from the absorbent layer also has the effect of degrading the surface of the belt or roller. Over time a continuously re-used belt or roller will pick up sufficient contaminants (e.g., paper fibers, dust, toner particles, etc.) to increase the surface energy. If the surface energy of the roller or belt increases, it will begin to adhere to surfaces that have a lower surface energy, like the photoreceptor, the intermediate transfer member, or even the toner. To keep contaminants from altering the surface energy of the roller or belt, a cleaning mechanism is frequently employed in an attempt to maintain integrity.
An irreversible problem associated with the drying rollers and belts of the prior art is when ozone from the corona in an electrophotographic printer oxidizes the surface of the roller or belt. Once ozone damage is done, there is no possibility for renewal.
The drying rollers of the prior art are expensive to make and difficult to exchange. They frequently have a metal core, adding to the cost of manufacture. Both belts and rollers are also consumable components of a printer that generally require a visit by a service person for exchange.
These and other problems associated with drying carrier liquid from an intermediate transfer member and/or drying carrier liquid from a liquid developed image on an intermediate transfer member are known to those skilled in the art. The art continually searches for solutions to these problems and improved drying methods.
This invention addresses problems associated with using a single absorbent roller, or absorbent belt to absorb excess carrier and a heating roll to remove the absorbed carrier so that the single absorbent roller or absorbent belt may be continually reused.
A first aspect of the invention is a liquid electrophotographic imaging apparatus containing at least one drying element for removing excess carrier liquid from a liquid toner toned latent image on an intermediate transfer member surface or the intermediate transfer member surface without the toner image. One element of an at least one drying sheet may be present as a flexible substrate having a first surface and second surface; another element is that there is at least one oleophilic carrier liquid absorptive layer on the first surface of the flexible substrate. The at least one drying sheet preferably should be distinct articles and is not a fixed layer or a fixed set of layers on a roller or belt. The at least one drying sheet preferably could be a series of single sheets, sheets on a roll with individual sheets defined by perforated separation lines, or a continuous sheet on a roll fed from one roll to another. Another element of the apparatus is that the first surface of the flexible substrate faces the intermediate transfer member surface, latent image or liquid toned latent image when it is placed in position to absorb liquid carrier. An alternative feature of the invention is that the first surface and said second surface both may have an oleophilic absorptive layer affixed to each of the first surface and the second surface. In one embodiment of the invention, the absorbent sheet may have an additional element of a compliant inner layer affixed between the flexible substrate and the at least one oleophilic carrier liquid absorptive layer. In a further embodiment of the invention, the inner layer may be oleophobic to carrier liquid. In a preferred embodiment, the inner layer comprises a polymer selected from nitrile elastomers, fluorosilicone polymers, fluorocarbon polymers, and polyurethane polymers.
In a preferred embodiment for the at least one absorbent layer, desirable materials include a polymer selected from the group consisting of silicone polymers, ethylene/propylene copolymers, polybutadienes, and polyisoprenes. The apparatus may provide sheet handling systems that move the drying sheet from a storage area or supply area to position the drying sheet into contact with a surface of an intermediate transfer member where the sheet will be able to contact carrier liquid for the purpose of drying liquid carrier. In a preferred embodiment, the absorbent layer of the sheet has a surface energy that is at least 1 dyne/cm less than the surface energy of the surface it is positioned to contact and to dry. In one feature, the imaging apparatus of this invention should provide a drying sheet that is capable of absorbing 2%-70% of its own weight in carrier liquid (e.g., 2-70% liquid carrier/98%-30% absorbent sheet). In one embodiment, the absorbent sheet has sufficient retention properties that the absorbent layer is a non-leaching absorbent. This means that the absorbent layer retains the carrier liquid with sufficient strength that ambient moisture and water in landfills will not remove solvent in an amount that would be prohibited by regulatory provisions. In one embodiment, a standard for absorption is where a non-leaching absorbent with 20% by weight carrier liquid (liquid/absorbent) buried in black dirt with 10% by weight water content, would lose 2% or less of the solvent (that is 0.4% of the weight of solvent plus absorbent) in a six-month period at 20xc2x0 C. and 40% relative humidity. In another embodiment, the absorbent layer is capable of absorbing carrier liquid from a freshly deposited liquid toner image and subsequently desorbing the carrier liquid upon application of heat or pressure. The absorbent layer may be continuous, completely solid, a matrix of materials, discontinuous or porous.
As noted earlier, the imaging apparatus may provide the absorbent sheet (which is capable of absorbing carrier liquid from an image and may be capable of subsequently desorbing the carrier liquid). In one embodiment, the sheets may be treated to desorb the absorbed liquid carrier either singly or in a cartridge upon application of heat or pressure.
As also noted earlier, the absorbent sheet may have an additional element of an inner layer which may be oleophobic. The absorbent sheet with the inner layer may additionally have any or all of the features and embodiments described above.
In another aspect of the invention, the electrophotographic imaging apparatus of the invention may be alternatively described as having the ability to remove excess liquid carrier from an intermediate transfer member surface. The electrophotographic imaging apparatus might then comprise a first element of an electrophotographic imaging system capable of providing an electrophotographic image on an intermediate transfer member; a second element consisting of an absorbent (liquid carrier absorbent) image drying sheet which contacts the intermediate transfer member, one feature of the image drying sheet having at least an outer layer which absorbs carrier liquid, one embodiment of the surface of the drying sheet in contact with the intermediate transfer member having a Shore A hardness of 10 to 60. One element of this aspect of the invention is that the drying sheet, after contacting the intermediate transfer member surface from which the toned image has been transferred, absorbs carrier liquid from a surface of the intermediate transfer member and the drying sheet then becomes a used drying sheet. In one embodiment, the apparatus preferably has a disbursing cartridge element for supplying non-saturated drying sheets for use and a receiving cartridge element for receiving used drying sheets. In another embodiment, the supply cartridge and the disbursing cartridge may be within a single housing. In an additional embodiment, there may be a heating element in the apparatus for evaporating carrier liquid from used drying sheets or a pressurizing zone for pressing liquid carrier from the absorbent sheet.
In another aspect of the invention, a method of drying or reducing the liquid carrier content of a liquid toner image or an intermediate transfer member surface may comprise steps such as providing at least one absorbent drying sheet and providing an electrophotographic apparatus. The electrophotographic apparatus should have at least an intermediate transfer member, at least one supply container and at least one discard container for the at least one absorbent drying sheet. Further steps include providing a toned image on the intermediate transfer member with a liquid toner; contacting an absorbent drying sheet from the supply container to the toned image on the intermediate transfer member or to the intermediate transfer member itself after the image is transferred away; and absorbing liquid carrier with the drying sheet, the drying sheet then becoming a used drying sheet. Various means can determine whether the used drying sheet is suitable for reuse as an absorbent drying sheet; and a final step might include placing the used drying sheet in a container selected from the group consisting of: supply container, re-supply container, regeneration container, or discard container depending upon the used drying sheet""s suitability for further use. A final step may alternatively include the use of a regeneration container wherein heat is applied to the used drying sheets causing at least a portion of the absorbed carrier to be expelled from the substantially saturated drying sheets thereby converting substantially saturated drying sheets to non-saturated drying sheets. A final step may also or in the alternative use a regeneration container wherein pressure is applied to the used drying sheets causing at least a portion of the absorbed carrier to be expelled from the substantially saturated drying sheets thereby converting substantially saturated drying sheets to non-saturated drying sheets. If a discard container is used as a final step, the container and/or the sheets maybe recycled or disposed of (if landfillability requirements are met, as discussed above) either before or after regeneration.
Another aspect of the invention is a method of removing carrier liquid from a liquid toner image on an intermediate transfer member or from an intermediate transfer member after transfer of a liquid toner image to a final substrate. Some of the steps in this process include providing a plurality of absorbent drying sheets in a cartridge, wherein the sheets are stacked such that there is a top of the stack and a bottom of the stack; and providing an electrophotographic apparatus comprising at least an intermediate transfer member and a cartridge of drying sheets. A toned image is provided on the intermediate transfer member; and in one embodiment, an absorbent drying sheet from the cartridge is contacted to the toned image on the intermediate transfer member. In another embodiment, an absorbent drying sheet from the cartridge is contacted to the intermediate transfer member itself after the image is transferred away, in either case the drying sheet after absorbing liquid carrier becoming a used drying sheet. Additional steps include replacing the used drying sheet at the top of the stack in the absorbent drying sheet cartridge for re-supply or discard. By using non-leachable absorbent layers and non-leachable absorbent sheets (as described above, such that the used or substantially saturated sheets meet current regulatory requirements), the cartridge may be removed when it is filled with used or substantially saturated drying sheets and may be disposed of in a landfill. Alternative final steps include applying heat to the used drying sheets causing at least a portion of the absorbed carrier to be expelled from the substantially saturated drying sheets thereby converting the cartridge of substantially saturated drying sheets to non-saturated drying sheets. A final step may also use pressure or in the alternative use pressure applied to the used drying sheets causing at least a portion of the absorbed carrier to be expelled from the used drying sheets thereby converting used drying sheets to non-saturated drying sheets
Another aspect of the present invention is also a method of drying carrier liquid from a toner image on an intermediate transfer member or from an intermediate transfer member after transfer to a final substrate. Some of the steps for this method include providing an electrophotographic apparatus comprising at least an intermediate transfer member and a continuous drying sheet. The continuous absorbent drying sheet has a beginning and an end, with the beginning attached to a take-up roller or spool and the end attached to a supply roller or spool in one embodiment, the intermediate portion thereof coiled around a supply spool with at least a portion of the sheet contacting the photoreceptor. Further steps include providing a toned image on the intermediate transfer member and contacting the continuous absorbent drying sheet to the toned image on the intermediate transfer member, or to the intermediate transfer member itself after the image is transferred away, creating a used portion of the continuous absorbent drying sheet; and simultaneously disbursing fresh length of the continuous absorbent drying sheet and taking-up the used portion of the continuous drying sheet. As discussed above, if the absorbent layer of the continuous absorbent drying sheet is non-leachable within regulatory standards, the discard or take-up spool may be landfilled. In another embodiment, the used drying sheet may be recycled.
These and other non-limiting aspects of the invention will be seen from the following examples.